This invention relates to metallic contacts for compound semiconductors. More particularly, the present invention relates to stable, single-phase, epitaxial metallic contacts for III-V compound semiconductors.
During the past two decades, considerable effort has been spent upon the development of metallic contacts for compound semiconductors. In general, workers in the art have focused upon three goals, namely, contact reproducibility and stability. Ideally, a contact that satisfies these criteria should consist of only a single phase that is epitaxial on the III-V compound semiconductor substrate, thereby eliminating the interphase boundaries and grain boundaries that are commonly associated with contact degradation and the lack of reproducibility. Furthermore, the contact material should not react with the compound semiconductor at elevated temperatures since reaction would lead to the formation of a new phase and, thus, interphase boundaries. Any reaction is also undesirable since the reaction will result in the consumption of compound semiconductor material, leading to an uncertainty in the final position of the contact/semiconductor interface and thus, contact non-reproducibility. An additional requirement is that of morphological stability; that is, the contact material should have a sufficiently high bond strength and melting point so that the contact film does not ball up or agglomerate during required thermal processing steps subsequent to contact deposition. For example, the contact must not agglomerate during the implant activation step (typically 800.degree. C. for several minutes or 950.degree. C. for several seconds) required when the contact is used an an implant mask. Likewise the contact must remain laterally uniform for hours at 500.degree.-700.degree. C. during the growth of a III-V semiconductor overlayer.
Unfortunately, none of the prior art efforts has succeeded in fabricating a contact that is single-phase, epitaxial, and stable against both film agglomeration and reactions with the III-V semiconductor substrate. Thus, for example, nominally monocrystalline films of aluminum have been grown on gallium arsenide by molecular beam epitaxy under ultra-high-vacuum conditions. However, the melting point of aluminum (660.degree. C.) is much too low to withstand thermal processing at elevated temperatures. Furthermore, exposure of aluminum to arsenic results in the formation of aluminum arsenide, thus preventing the overgrowth of III-V semiconductors such as gallium arsenide on films of aluminum without the transformation of the metallic aluminum to semiconducting or insulating aluminum arsenide.
Still further efforts to effect the desired end involved the deposition of transition metals such as nickel, cobalt and iron upon gallium arsenide under ultra-high vacuum conditions. Although such techniques resulted in the formation of films evidencing a high degree of crystallographic texture, none of these metals proved stable on gallium arsenide since moderate annealing at temperatures above 400.degree. C. is sufficient to produce ternary and binary product phases. In related work it has been demonstrated that reacting thin films of nickel or cobalt with gallium arsenide does produce stable binary phases that are epitaxial. However, undesirable, nonepitaxial nickel-arsenide and cobalt-arsenide phases are formed as well. The fact that these reactions consume the III-V substrate is undesirable from a contact reproducibility perspective. Furthermore, these two-phase films agglomerate at elevated temperatures, leading to laterally nonuniform contacts with poor electrical properties. In addition, the partially non-epitaxial nature of the two-phase films makes these contacts unsuitable as templates for compound semiconductor overgrowth.